This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Weiliu, you're getting unexpected result because hash set does not guarantee the order of its elements because it's based on hashing. If you want to enforce the results, you can use TreeSet. Best regards, -Asif